1. Field of the Invention
The present invention relates to an encoder and, more particularly, to an encoder for detecting a relative displacement between a diffraction grating and a light beam incident on the diffraction grating by causing several diffracted components emerging from the diffraction grating to interfere with each other and photoelectrically converting an interfered beam.
2. Related Background Art
A conventional encoder is used in an NC machine tool or the like as a sensor for detecting the position or an angular displacement of a workpiece. In recent years, a higher resolution and higher precision are increasingly required for such an encoder.
In a known conventional high-precision encoder having a high resolution, a diffraction grating is used as a displacement detection optical scale, a recording density of the diffraction grating is set to be several microns per pitch, and several diffraction light components emerging from the diffraction grating are interfered with each other to obtain a period signal corresponding to a displacement of the scale. When the recording density of the diffraction grating is, however increased to increase the resolution and improve the precision, e.g., to an order of the light wavelength, a diffraction angle (i.e., an angle of a beam emerging from the diffraction grating) of the diffracted beam is increased, and the layout of optical members becomes cumbersome.
In a conventional encoder shown in FIG. 7, a signal corresponding to a displacement of a scale is generated by the following operations.
A light beam from a laser diode 1 is collimated by a collimator lens 2 and is vertically incident on a point P1 on a diffraction grating 5. A first mode diffracted beam (R1+) emerging from the point P1 is returned to a beam splitter 4 through a mirror 61. At the same time, a first mode diffracted beam (R1-) emerging from the point P1 is returned to the beam splitter 4 through a mirror 62. These first mode diffracted beams are interfered with each other through the beam splitter 4. The phase of the wave front of the positive first mode diffracted beam is advanced by 2.pi. and the phase of the wave front of the negative first mode diffracted beam is delayed by 2.pi. while the diffraction grating 5 is moved by one pitch. On the basis of this principle, the two beams are interfered with each other to obtain an interfered beam. An intensity change having two periods upon movement of one pitch of the grating can be obtained by this interfered beam. That is, a period signal having a period twice the number of grating lines of the diffraction grating 5 can be extracted.
As described above, however, when the recording density of the diffraction grating 5 is increased (pitch is decreased), the diffraction angle of the diffracted beam is increased. The exit angle of the diffracted beam from the diffraction grating 5 is close to 90.degree. . The mirrors 61 and 62 must be located near the diffraction grating 5 so as not to be brought into contact with the diffraction grating 5. This layout is very cumbersome. In addition, when the grating pitch of the diffraction grating 5 is smaller than the wavelength of the beam from the laser diode 1, diffracted components cannot be extracted from the diffraction gratings. In this case, it is impossible to detect even a change in diffraction grating 5.
The present assignee disclosed a rotary encoder (copending application U.S. Ser. No. 522,051) which could solve the above problem and could easily extract diffracted components even if the pitch of the diffraction grating is decreased.